An important aspect of the manufacture, regulatory review and approval of all dosage forms concerns their stability over extended periods of time. The stability data obtained with regard to a particular dosage form directly affects its shelf-life. The stability of a pharmaceutical dosage form is related to maintaining its physical, chemical, microbiological, therapeutic, and toxicological properties when stored, i.e., in a particular container and environment. Stability study requirements are covered, e.g., in the Good Manufacturing Practices (GMPs), the U.S.P., as well as in the regulatory requirements of the country where approval to market a dosage form is being sought. In the United States, a request to test, and eventually market, a drug or a drug formulation may be made via a New Drug Application (NDA), an Abbreviated New Drug Application (ANDA) or an Investigational New Drug Applications (IND).
The agents used in sustained release dosage formulations often present special problems with regard to their physical stability during storage. For example, waxes which have been used in such formulations are known to undergo physical alterations on prolonged standing, thus precautions are taken to stabilize them at the time of manufacture or to prevent the change from occurring. Fats and waxy materials when used in purified states are known to crystallize in unstable forms, causing unpredictable variations in availability rates during stability testing at the time of manufacture and during later storage.
It is known that certain strategies can be undertaken to obtain stabilized controlled release formulations in many cases, such as insuring that the individual agents are in a stable form before they are incorporated into the product, and that processing does not change this condition, retarding the instability by including additional additives, and inducing the individual agents of the dosage form to reach a stable state before the product is finally completed.
It is also recognized that the moisture content of the product can also influence the stability of the product. Changes in the hydration level of a polymeric film, such as the ethyl celluloses, can alter the rate of water permeation and drug availability. Also, binders such as acacia are known to become less soluble when exposed to moisture and heat. However, moisture content of a product can be controlled fairly successfully by controls in the processing method and proper packaging of the product.
Hydrophobic polymers such as certain cellulose derivatives, zein, acrylic resins, waxes, higher aliphatic alcohols, and polylactic and polyglycolic acids have been used in the prior art to develop controlled release dosage forms. Methods of using these polymers to develop controlled release dosage forms such as tablets, capsules, suppositories, spheroids, beads or microspheres are to overcoat the individual dosage units with these hydrophobic polymers. It is known in the prior art that these hydrophobic coatings can be applied either from a solution, suspension or dry. Since most of these polymers have a low solubility in water, they are usually applied by dissolving the polymer in an organic solvent and spraying the solution onto the individual drug forms (such as beads or tablets) and evaporating off the solvent.
Aqueous dispersions of hydrophobic polymers have been used in the prior art to coat pharmaceutical dosage forms for aesthetic reasons such as film coating tablets or beads or for taste-masking. However, these dosage forms are used for immediate release administration of the active drug contained in the dosage form.
The use of organic solvents in the preparation of hydrophobic coatings is considered undesirable because of inherent problems with regard to flammability, carcinogenicity, environmental concerns, and safety in general. It considered very desirable in the art, however, to provide a controlled release coating derived from aqueous dispersions of a hydrophobic material, such as ethyl cellulose.
Unfortunately, to date, while many formulations have been experimentally prepared which rely upon a hydrophobic coating derived from an aqueous dispersion to provide controlled release of an active agent, such formulations have not proven to be commercially viable because of stability problems. Aqueous polymeric dispersions have been used to produce stable controlled release dosage forms, but this has only been possible by other methods such as incorporation of the same into the matrix of the dosage form, rather than via the use of a coating of the aqueous polymeric dispersion to obtain retardant properties.
When coating using aqueous polymeric dispersions to obtain a desired release profile of the active agent(s) over several hours or longer, it is known in the art that the dissolution release profile changes on ageing, e.g. when the final coated product is stored for a period of time, during which time it may be exposed to elevated temperature and/or humidity above ambient conditions. This problem is illustrated, for example, by Dressman, et al., Proceed. Intern. Symp. Control. Rel. Bioact. Mater., 18 (1991), pp. 654-655, Controlled Release Society, Inc. who reported that phenylpropanolamine HCl pellets coated with an ethyl cellulose-based film are only stable at room temperature under ambient humidity conditions. Munday, et al., Drug Devel. and Indus. Phar., 17 (15) 2135-2143 (1991) similarly reported that film coated theophylline mini-tablets film coated with ethyl cellulose with PEG (2:1), and ethyl cellulose with Eudragit L (2:1) impeded dissolution upon storage under stress conditions, the degree of slowdown of release being said to be directly proportional to temperature, while the effect of relative humidity (RH) appeared to be insignificant. Munday, et al. concluded that the decreased rate of release was due to the slowing in the rate of molecular diffusion of the drug across the polymeric coating material, and suggested that the change was due to significant alterations in the permeability of the polymer which occurred during the experimental storage.
This instability problem is known to not exist when the polymers are applied from organic solvent solution.